This invention relates to a method of preparing polycarbonate. More particularly the method relates to a method whereby a solution comprising a solvent and an oligomeric polycarbonate is introduced into a devolatilizing extruder wherein the oligomeric polycarbonate is converted into high molecular weight polycarbonate while simultaneously removing the solvent. More particularly, the instant invention relates to the formation under mild conditions of polycarbonates having extremely low levels of Fries rearrangement products, a high level of endcapping and low levels of residual solvent.
Polycarbonates, such as bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.
The interfacial method for making polycarbonate has several inherent disadvantages. First it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chloride content, which can cause corrosion.
The melt method, although obviating the need for phosgene or a solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer""s flow properties and performance. The melt method further requires the use of complex processing equipment capable of operation at high temperature and low pressure, and capable of efficient agitation of the highly viscous polymer melt during the relatively long reaction times required to achieve high molecular weight.
Some years ago, it was reported in U.S. Pat. No. 4,323,668 that polycarbonate could be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diaryl carbonate formed by reaction phosgene with methyl salicylate. The method used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve high molecular weight polycarbonate. High catalyst loadings are particularly undesirable in melt polycarbonate reactions since the catalyst remains in the product polycarbonate following the reaction. The presence of a transesterification catalyst in the polycarbonate may shorten the useful life span of articles made therefrom by promoting increased water absorption, polymer degradation at high temperatures and discoloration.
In copending U.S. applications Ser. Nos. 09/911,439 and 10/167903, now issued as U.S. Pat. Nos. 6,420,512 and 6,506,871 respectively, extrusion of a mixture of an ester-substituted diaryl carbonate, such as bis-methyl salicyl carbonate, a dihydroxy aromatic compound, such as bisphenol A, and a transesterification catalyst, such as tetrabutylphosphonium acetate (TBPA), afforded high molecular weight polycarbonate. The extruder employed was equipped with one or more vacuum vents to remove by-product ester-substituted phenol. Similarly, a precursor polycarbonate having ester-substituted phenoxy endgroups, for example methyl salicyl endgroups, when subjected to extrusion afforded a polycarbonate having a significantly increased molecular weight relative to the precursor polycarbonate. The reaction to form a higher molecular weight polycarbonate may be catalyzed by residual transesterification catalyst present in the precursor polycarbonate, or by a combination of any residual catalyst and an additional catalyst such as TBPA introduced in the extrusion step. Fries rearrangement products were not observed in the product polycarbonates.
Although the methods described in copending U.S. application Ser. Nos. 09/911,439 and 10/167,903 (now issued as U.S. Pat. Nos. 6,420,512 and 6,506,871 respectively) represent significant enhancements in the preparation of polycarbonate relative to older methods, additional improvements are needed. For example, it would be highly desirable to increase the throughput rate of starting materials through the extruder in order to achieve greater efficiency. In addition, it would be highly desirable to avoid having to isolate a precursor polycarbonate having ester-substituted phenoxy endgroups prior to its extrusion to afford a higher molecular weight polycarbonate.
The present invention provides a method for the preparation of polycarbonate, said method comprising extruding in the presence of a transesterification catalyst at one or more temperatures in a temperature range between about 100xc2x0 C. and about 400xc2x0 C. a solution comprising a solvent and an oligomeric polycarbonate, said extruding being carried out on an extruder equipped with at least one vent adapted for solvent removal, said oligomeric polycarbonate comprising polycarbonate repeat units derived from at least one dihydroxy aromatic compound, said oligomeric polycarbonate comprising ester substituted phenoxy terminal groups having structure I 
wherein R1 is a C1-C20 alkyl group, C4-C20 cycloalkyl group, or C4-C20 aryl group; R2 is independently at each occurrence a halogen atom, cyano group, nitro group, C1-C20 alkyl group, C4-C20 cycloalkyl group, C4-C20 aryl group, C1-C20 alkoxy group, C4-C20 cycloalkoxy group, C4-C20 aryloxy group, C1-C20 alkylthio group, C4-C20 cycloalkylthio group, C4-C20 arylthio group, C1-C20 alkylsulfinyl group, C4-C20 cycloalkylsulfinyl group, C4-C20 arylsulfinyl group, C1-C20 alkylsulfonyl group, C4-C20 cycloalkylsulfonyl group, C4-C20 arylsulfonyl group, C1-C20 alkoxycarbonyl group, C4-C20 cycloalkoxycarbonyl group, C4-C20 aryloxycarbonyl group, C2-C60 alkylamino group, C6-C60 cycloalkylamino group, C5-C60 arylamino group, C1-C40 alkylaminocarbonyl group, C4-C40 cycloalkylaminocarbonyl group, C4-C40 arylaminocarbonyl group, or C1-C20 acylamino group; and b is an integer 0-4.
The present invention further relates to a method for preparing solutions comprising an ester substituted phenol solvent and an oligomeric polycarbonate, and the conversion of said oligomeric polycarbonate into high molecular weight polycarbonate with simultaneous removal said solvent, said method comprising:
Step (I) heating a mixture comprising at least one dihydroxy aromatic compound, an ester substituted diaryl carbonate and a transesterification catalyst at a temperature in a range between about 100xc2x0 C. and about 300xc2x0 C. to provide a solution of an oligomeric polycarbonate in an ester substituted phenol solvent; and
step (II) extruding said solution of oligomeric polycarbonate in said ester substituted phenol at one or more temperatures in a range between about 100xc2x0 C. and about 400xc2x0 C., and at one or more screw speeds in a range between about 50 and about 1200 rpm, said extruding being carried out on an extruder comprising at least one vent adapted for solvent removal.
In another aspect the present invention relates to a polycarbonate prepared according to the method of the invention, said polycarbonate having a very high level of endcapping, a very low level of Fries product, and a very low level of residual solvent.